Prior art protective films for semiconductor devices and dielectric films for multilayer printed wiring boards utilize photosensitive polyimide, epoxy and silicone resin compositions. While many photosensitive materials are known for the protection of such substrates and wiring boards, JP-A 2002-088158 proposes a photosensitive silicone composition having the advantage of flexibility. Although this photosensitive silicone composition cures at low temperature to form a film which is reliable due to improved moisture-resistant adhesion, it has poor chemical resistance in photoresist strippers having a high dissolving power such as N-methyl-2-pyrrolidone.
JP-A 2008-184571 discloses a photosensitive silicone composition based on a silphenylene structure-bearing silicone polymer. Although this photosensitive silicone composition has improved chemical resistance in liquid photoresist strippers, it is desired to have more chemical resistance in solder flux fluid and higher reliability as the semiconductor device protective film.
Also, JP-A 2008-026660 proposes a photosensitive resin composition comprising, in combination, a polyfunctional epoxy resin, a phenolic curing agent, and a photoacid generator in the form of a phosphate anion-containing sulfonium salt. This resin composition can form a pattern having a high aspect ratio, but suffers from a substantial internal stress. It is desired to tailor the resin composition to be more flexible.
For the purpose of forming patterns using photosensitive resins, development works are widely made on a radical polymerization system utilizing a photo-radical generator for chain polymerization of vinyl groups and a cation polymerization system utilizing a photoacid generator. These systems, however, have several problems. First in the radical polymerization system, since the reaction is inhibited by airborne oxygen, a special measure for shutting off oxygen is necessary. In the cation polymerization system, strong acid generated is neutralized with a minute amount of airborne basic substance, whereby the pattern profile is degraded. If the generated acid is left even after curing, it causes corrosion to semiconductor devices or modifies the cured film to detract from its properties.